Find a segment for that area. Find multiple people with the same speed/time on that day, bingo. There's your group ride. Snoop their Strava clubs and do some Google snooping. Send a nice message to their admin on their local site.
I've done this before when I came across a critical mass group run on a local MUP. No issues with that. Issue was they took up the entire width of the trail and were strung out for a 1/4 mile, half wearing headphones. No attention to bells to calls.
I did that because I use the MUP that day of the week and time pretty often. It's the only safe way to cross a super busy part of town to get to somewhere I'm going.
The organizer was quite apologetic and promised to bring it up at announcements at the next run. I would totally expect them to contact our bike group if we had an incident where a biker did something stupid. We want to know so the group as a whole isn't looked at with disdain in the community.